Bootlace worms may grow very long but are usually only 5 to 10 millimetres (0.20 to 0.39 in) in width. The body is brown with lighter (longitudinal) stripes.[3] Its mucus contains a relatively strong neurotoxin which it uses as a defense against predators. When handled, it produces large amounts of thick mucus with a faint pungent smell, reminiscent of iron or sewage.[4] This toxic mucus has been shown to kill crabs and cockroaches, and could have applications as an agricultural insecticide.[5]

In 1864 a specimen washed ashore in the aftermath of a severe storm by St Andrews, Scotland, which was more than 55 m (180 ft) long,[6] longer than the longest known Lion's mane jellyfish, the animal which is often considered to be the longest in the world. However, records of extreme length should be taken with caution, because the bodies of nemerteans are flexible and can easily stretch to much more than their usual length.

Like other nemerteans, Lineus longissimus feeds using its eversibleproboscis. As it is in the class Anopla, its proboscis is not armed with a barbed stylet. Instead, it has a cluster of sticky filaments at the end of its proboscis that it uses to immobilize prey.